The shaping abilities of nickel-titanium K-files, stainless steel reamers, K-files, and flexible stainless steel instruments with conventional cutting tips and with modified noncutting tips were investigated under standardized conditions using a computer-driven testing device simulating the clinical use of the instruments. Simulated root canals with a 42-degree curvature were sequentially enlarged from #15 to #35. Undesirable changes in the canal shape as a result of instrumentation occurred in all cases. None of the instruments were able to remove material on the whole length of the inner side of the curvature, whereas all instruments removed material on the whole length of the outer side of the curvature, resulting in slight to severe bulging. The extent of undesirable changes in the canal shape depended typically of the type of instrument used. Best instrumentation results were obtained with flexible instruments with noncutting tips.